The concerned reactions generally develop reaction heat, thereby requiring one or more intermediary thermal adjustments with an external medium.
It is known that chemical reactions developing much heat may be performed in several steps with an intermediary thermal adjustment after each step.
Thus, the catalytic reforming of gasolines, for example, (See French patent No. 2,160,269, U.S. Pat. No. 4,210,519 and U.S. Pat. No. 4,233,268) is usually conducted in three reactors with solid catalyst beds separated by two external heating furnaces.
The use of so many reactors is expensive in equipment, pipes and installation costs.
For this reason it has often be proposed to arrange the different reactors in a single reaction enclosure whose walls are adapted to withstand the relatively high internal pressure of the system. For this purpose, the solid catalyst beds are vertically superposed and lay on supporting grids or directly on horizontal partition walls (French Pat. No. FR 2,573,996).
These grids and partitions must withstand both the weight of the catalyst bed and the strains resulting from the pressure drop generated by the flow of reaction fluid through heat-transfer elements and through the catalyst bed itself.
These two strains result in extremely high totals of the order of 50-100 or even 150 tons per square meter, whereas the current specifications for industrial floors are limited to about 0.5-1 ton per square meter.
In order to avoid this cumulative strain effect resulting in constraining mechanical solutions as concerns as well the weight of the supporting beams as the lost dead space, it has been proposed, particularly for methanol reactors (Hydrocarbon Processing, May 1984 p. 95-100) to superpose reactors, not of the axial but of the radial type, where the catalyst is placed into a hollow cylindrical cartridge, or even into several cartridges, and the reaction fluid flows horizontally, either centrifugally from the internal cylinder to the external cylinder or centripetally from the external cylinder to the internal cylinder.
This solution has the advantage of releasing the separation floors from the strains due to the reaction fluid circulation, but has the disadvantage of leaving a large dead space, particularly a void central core which, according to British Patent No. 1,140,071 can be used to house a heat exchanger.
A disadvantage of this solution, emphasized in U.S. Pat. No. 4,225,562, results in the difficulty of accurately centering the two cylinders defining the ring of solid catalyst. A bad centering results in a heterogeneity of the fluid flow paths. Such a result is highly detrimental to good performance of the main reaction and results in excessive temperatures which are dangerous for the catalyst stability.
Another disadvantage is the requirement of charging and discharging catalyst cartridges. For this purpose, flanges of the same diameter as that of the reactor can be provided, but this is not the best solution when operating under pressure and with a reactor of large size. It is then recommended to weld the generally hemispherical end parts to the cylindrical body. But, for each catalyst discharge operation it is necessary to saw off said end parts to withdraw the cartridges and to then weld back said parts again after the reactor charge phase. The operation is time-consuming, delicate and requires at each time administrative authorization and control.
On the other hand U.S. Pat. No. 4,225,562 teaches the provision of compartments, shaped as parallelepiped arranged parallel to the enclosure axis all of these compartments have the same sectional area and volume.
The device uses to the best one half of the available space. In addition to the mechanical complexity and the corresponding high cost of the device, the standardization of compartments of identical volume precludes the adaptation of the catalyst volume to the variation of the reaction velocity.
Consequently, the device according to this patent can be used only with reactors either of the single stage type, as far as kinetics is concerned, or performing reactions whose velocity is independent from the conversion rate, i.e., reactions of zero order.
Among the various embodiments of reactors with radial partitions, it is worth to mention the device disclosed in U.S. Pat. No. 3,898,049, wherein the catalyst cartridge is divided into several sectors in a longitudinal direction. The reaction fluids successively pass through these sectors, alternately downwardly and upwardly, in the direction of the reaction enclosure longitudinal axis. It is easily observed that the proposed device is only applicable in very peculiar circumstances where the travel lengths, and hence the pressure drops, are unimportant since, with reference to an axial reactor of same height and, a fortiori, to a radial reactor, the fluid travel is considerably extended in proportion to the number of sections of the cartridge.
The prior art is further illustrated by the British Patent No. 2,120,119 disclosing a longitudinal reactor for conducting chemical syntheses in gas phase. This reactor comprises catalyst-containing parallepiped enclosures arranged along the reactor PG,6 axis and having permeable opposite walls wherethrough the charge circulates. The effluent may be innerly cooled by quench, this limiting the available internal space.
French Patent No. FR 2,573,996 illustrates a catalytic reactor with only one compartment for ammonia and methanol synthesis without internal heat exchanger. The wall temperature is maintained at a low level by the presence of an air space.
German Patent No. DE 2,929,300 discloses an exchanger reactor with axial flow through catalyst zones of variable cross-section, these zones being non-adjacent.